Clostridial neurotoxins (CNT) are the most toxic substances known. There are eight related toxins—seven botulinum neurotoxins (BoNT/A-G) and tetanus neurotoxin (TeNT) (Schiavo et al., 2000; Simpson, 1981). BoNTs can cause botulism disease and are potential biological weapons (Amon et al., 2001; Mahant et al., 2000). Each of the BoNT and TeNT is composed of a heavy and light chain; the heavy chain mediates binding to the surface of specific nerve terminals. Once internalized via endocytosis, the light chain is translocated from the lumen of the vesicle into the cytoplasm where it functions as a zinc-dependent protease (Schiavo et al., 2000). The light chain cleaves one or more components of a conserved membrane fusion complex composed of syntaxin, SNAP-25 and synaptobrevin (syb), thereby blocking exocytosis (Blasi et al., 1993a; Blasi et al., 1993b; Schiavo et al., 1992; Schiavo et al., 1993). Because of their ability to selectively disrupt Ca2+-triggered exocytosis, the CNTs have emerged as important tools for the study of membrane fusion and synaptic transmission (Jahn and Niemann, 1994).
The first step in the action of CNTs involves binding to receptors on the surface of neurons. Current evidence suggests that the receptors are composed of gangliosides and proteins that cooperate to form high affinity toxin binding sites. Alternatively, gangliosides may constitute relatively low-affinity toxin binding sites that serve to capture CNTs to facilitate interactions with cell surface receptor proteins (Montecucco, 1986; Nishiki et al., 1996a). Gangliosides are ubiquitous glycosphingolipids in the outer leaflet of plasma membranes. They are classified according to the number and position of sialic acids present in their head groups. Polysialiogangliosides, which are present almost exclusively in neurons and neuroendrocrine cells, bind to CNTs with the greatest avidity (Halpern and Neale, 1995). While a protein component is also clearly involved in toxin-cell recognition, at present, a protein that mediates toxin entry has not been identified (Schiavo et al., 2000).
Biochemical studies have led to the identification of a handful of CNT binding proteins. In most case, these binding proteins do not appear to function as receptors that mediate entry of the toxins. For example, BoNT/A,B,E and TeNT were reported to bind synapsin I and adducin, respectively (Schengrund et al., 1996; Schengrund et al., 1993; Schengrund et al., 1992). Since neither of these proteins are exposed to the outside surface of cells, they are unlikely to function as cell surface receptors. TeNT was reported to bind Thy-1, a GPI-anchored plasma membrane protein. However, neurons from mice lacking Thy-1 are still sensitive to TeNT, suggesting that Thy-1 is not essential for TeNT entry into cells (Herreros et al., 2001).
Synaptotagmins (syt) I and II (Nishiki et al., 1994) are homologous synaptic vesicle membrane proteins thought to function as Ca2+-sensors for exocytosis (Chapman, 2002; Schiavo et al., 1998). Syt I and II were reported to bind BoNT/B in the presence of gangliosides; the dissociation constant for the syt I.BoNT/B complex was 2.3 nM and the dissociation constant for syt II.BoNT/B was 0.23 nM (Nishiki et al., 1996a). High affinity binding of BoNT/B to fibroblasts was reconstituted by expression of syt II and incorporation of exogenous gangliosides into surface membranes. However, binding did not result in the cleavage of the BoNT/B target protein, syb II, that had been co-expressed with syt II, indicating that the toxin was not internalized (Nishiki et al., 1996b). Although biochemical studies clearly established that syt binds to BoNT/B, evidence that binding mediates entry into cells is lacking. Thus, whether this interaction has any functional role remains unknown. More recently, BoNT/A and E have also been reported to bind syt I, albeit in a ganglioside independent manner (Li and Singh, 1998).
Syt II is a 422-amino acid protein that contains a luminal domain (a.a. 1-60), a transmembrane domain (a.a. 61-87) and a cytoplamic domain (a.a. 88-422). The cytoplasmic domain contains two C2 domains: C2A (a.a. 88-267) and C2B (a.a. 275-422) linked by a linker region (a.a. 268-274).
Determining whether any of the above proteins, or perhaps other proteins, serve as the BoNT receptor will be extremely useful for designing molecules that can reduce or completely inhibit BoNT toxicity. For the same reason, once a receptor is identified, it is important to map the BoNT binding domain because polypeptides containing the domain and peptidomimics thereof can be used to compete with the receptor for BoNT binding, thereby reducing or completely inhibiting BoNT toxicity.